1. Field of the Invention
This invention relates in general to high temperature superconductors, and, more particularly, to high temperature superconductor yarns and wires and to a process of making same.
2. Description of Related Art
Numerous investigations have been made to develop flexible wires, tapes, ribbons and yarns. However, success has been elusive because of the inherent brittleness of high temperature superconductor compositions, including YBa.sub.2 Cu.sub.3 O.sub.7-x which is referred to herein as a 123 superconductor. It is known to synthesize 123 superconductors by many different methods. The principal requirement being that yttrium oxide, barium oxide and copper oxide in appropriate proportions and mixed thoroughly, must be subjected to elevated temperatures for a certain length of time to form the correct superconductor composition and crystal structure. Thus, high temperature superconductor synthesis and procedures to sinter and compact the 123 superconductor powders to produce the now famous Meissner effect are well known to those skilled in the art.
While high temperature superconductor powders are ordinarily produced by mixing the respective oxides of Y, Ba and Cu, many alternative approaches have been suggested. Instead of using the oxides, it is possible to produce the same via oxalates, nitrates, and citrates of the respective elements (Y, Ba and Cu) in liquid forms to attain more homogenous mixtures. When subjected to high temperatures, these precursors decompose and the resulting oxides react to form the superconductor composition. Most of these processes yield superconductor powders of comparable quality and performance insofar as the T.sub.c and J.sub.c is concerned. Regardless of the powder source, during processing to convert the powders to a the superconductor product, defects are often introduced, however, inadvertently, which results in poorer than expected electrical properties. On the other hand, oriented single crystal thin films of 123 exhibit J.sub.c values of well over 10.sup.6 A/cm.sup.2.
Products prepared from polycrystalline powders are naturally characterized by many grain boundaries. Of course, the short coherence length (15 Angstroms) in these superconductors adds to the difficulty in processing wires with suitable electrical properties. Silverizing via AgNO.sub.3 has been fairly successful to improve the electrical properties, but the J.sub.c, in particular, needs to be improved at least by an order of magnitude to 10,000 A/cm.sup.2 at a minimum, and preferably to 100,000 A/cm.sup.2 at 77k.
Alignment of grains in a polycrystalline high temperature superconductor product is mandatory. Plastic deformation is a known possible mechanism to align the grains. Magnetic alignment is also possible but these superconductors do not exhibit ferromagnetism. They are weakly magnetic and extremely strong magnetic fields are needed to induce alignment of powdered grains in the preferred (a-b plane). More importantly, any further processing, e.g., drawing, swaging, extrusion, etc. must be done without disturbing alignment achieved through magnetic means. Most of these wire treating processes are likely to rearrange grains during deformation, and reduce or damage the electrical properties. Typically, the low temperature superconductor (LTS) wires such as NbTi, Nb.sub.3 Sn, Nb.sub.3 Ge embedded in a copper matrix are 5-15 microns in diameter.
Conventionally, LTS wires are prepared via plastic deformation. For example, NbTi rods of a particular diameter, (for example, 1/8") are inserted in a copper billet. This billet is then hot extruded to a smaller diameter, causing the NbTi rods to elongate together, and by successive extrusion, it is possible to reduce the diameter to very fine dimensions. The mechanical properties of Cu and NbTi are such that both elongate or deform uniformly. The 1/8" diameter rod of NbTi can be reduced to as fine a 5-15 microns. However, none of the processes known in the prior art are capable of producing very fine diameter wires resembling low temperature superconductors. As a consequence, users of these wires would be more inclined to apply high temperature superconductor wire resembling low temperature superconductor configurations known at the present lime, so that the design of their current equipment would not require radical modification.
It is an object of the present invention to provide a superconducting wire and process of making same.
Another object of the invention is to provide a superconducting yarn and process of making same.